Detector alignment board

ABSTRACT

An alignment board for interfacing arrays of infrared detectors to a multi-layer module concerns an insulating board having a multiplicity of conductive vias with insulating sealing plugs and enlarged metallic pads that are attached to the ends of the vias. Preferably, an insulating layer is formed on opposing sides of the board. Openings are delineated in the layer coinciding with the metallic pads, and preferably solder contacts are deposited in the openings. A multi-layer module may be attached to one side of the alignment board using reflow solder processes. Then, arrays of infrared detectors may be attached to the opposing side of the alignment board using a lower-temperature reflow soldering process. The alignment board facilitates the interfacing and assembly of the focal plane of infrared detection systems.

FIELD OF THE INVENTION

The present invention relates generally to interfacing arrays ofinfrared detectors to integrated circuit modules, and more particularlyto an alignment board for interfacing detectors to a multi-layer modulehaving layers disposed transverse to the plane of the detectors.

BACKGROUND OF THE INVENTION

Space-based surveillance systems use infrared detectors coupled tocomputerized data processors for monitoring heated objects and theirmovements in the atmosphere below and on the ground. The function ofinfrared detectors is to respond to energy of a wave length within someparticular portion of the infrared region. Heated objects dissipatethermal energy having characteristic wave lengths within the infraredspectrum. The infrared spectrum covers a wide range of wave lengths,from about 0.75 micrometers to 1 millimeter. Different levels of thermalenergy, corresponding to different sources of heat, are characterized bythe emission of signals within different portions of the infraredfrequency spectrum. Detectors are selected in accordance with theirsensitivity in the range of interests to the designer. Similarly,electronic circuitry that receives and processes the signals from theinfrared detectors is also selected in view of the intended detectionfunctions.

Current infrared detection systems incorporate arrays of large numbersof discrete, highly sensitive detector elements the outputs of which areconnected through sophisticated processing circuitry. It is difficult,however, to actually construct structures that make a million or moredetector elements and associated circuitry in a reliable and practicalmanner. Practical applications for contemporary infrared detectionsystems necessitate that further advances be made in the reliability andeconomical production of assemblies of detector arrays and accompanyingcircuitry.

Because the array material is very thin, less than 0.005 inches,difficulties arise in attaching the array material to the base of themodule. One such difficulty is the inability of the detector material toabsorb forces generated by the mismatched coefficient of expansionbetween the module and the array material. An additional difficultyencountered is providing a means for testing the reliability of theindividual detector elements. Where the detector material is applieddirectly to the module body it is difficult to isolate a fault that maybe attributable to either the detector elements, module wiring orprocessing elements. Schmitz, U.S. Pat. No. 4,792,672, also assigned toGrumman Aerospace Corporation, addressed those issues with a bufferboard disposed intermediate the detector array segment and themulti-layer module. The buffer board facilitates assembly of thedetectors to the multi-layer module, and also enhances the structuralcharacteristics and separate testability of the system components.

Considerable difficulties are also presented in aligning the detectorelements with conductors on the connecting module and in isolatingadjacent conductors in such a dense environment. Indium bumps andflip-chip bump bonding techniques are commonly used for high-densityinterconnection in integrated circuitry, such as here between infrareddetector rays and signal processing modules. Indium bumps 30 to 40microns in diameter and spaced approximately 100 microns apart (centerto center) are typically formed in arrays upon two substrate surfaces tobe electrically connected, such that the indium bumps will fuse whenbrought into contact and forced together. Each indium bump may beconnected to a conductive conduit which provides electricalcommunication to integrated circuitry formed upon the substrate. In thepresent application the opposing substrate supports an infrared detectorarray containing 1000 or more detector pixels are formed upon asemiconductor substrate, and must be electrically connected to signalconditioning electronics formed upon the other semiconductor substrate.

The conventional positioning of detector arrays relative to multi-layermodules for attachment and electrical connection is a difficult process.Careful positioning and a means to maintain the position under varioustemperature conditions is required. Small sections of detectors arepositioned and held by precision tooling, until a bonding media such asepoxy can cure to form a permanent structure. Epoxy materials mayshrink, or absorb moisture and expand, changing the position of thedetectors.

Although prior art practices of forming indium contact bumps and bondingarrays of infrared detectors to signal processing modules have provengenerally suitable for their intended purposes, they possess inherentdeficiencies which detract from their overall effectiveness in themarketplace. In view of the shortcomings of the prior art, it isdesirable to provide a method for bonding integrated circuit modulestogether with bump contacts which is simpler to practice and which has ahigher yield than contemporary processes. It is also desirable tocircumvent the use of adhesives during the positioning of the detectorarrays, and instead rely on another method of manufacture.

SUMMARY OF THE INVENTION

The present invention specifically addresses and alleviates theabove-mentioned deficiencies associated with the prior art. Generally,the present invention comprises an alignment board for positioningarrays of infrared detectors relative to multi-layer integrated circuitmodules for structural attachment and electrical connection. Thestructural and electrical connections are preferably made using reflowsoldering processes, and the alignment board becomes part of the finalassembly of the infrared detection system. The design of the alignmentboard includes metallized via interconnects with insulating materialsealing plugs contained therein. Metallic pads are added on preferablyboth ends of the vias to enlarge the area for bump bonding, making thealignment easier.

More particularly, the preferred embodiment of the alignment board ofthe present invention is comprised of a thin yet stiff insulatingmaterial, through which a multiplicity of through-holes are located.Conductive vias are formed in the through-holes, having a preferablygold coating attached to the inside wall of the through-holes. Thealignment board further includes preferably a glass material within theconductive vias, to form sealing plugs. The sealing plugs help protectthe vias from damage during subsequent manufacturing operations. It isalso desirable not to leave voids in the alignment board, which couldlead to a collecting area for contaminants, or an outgassing problemwhen the alignment board is used for space-based applications.

The alignment board further includes a multiplicity of thin metallicpads that are structurally formed on and electrically connected topreferably both of the opposing ends of the vias. The diameter of themetallic pads is preferably substantially larger than the diameter ofthe opposing ends of the via, such that the metallic pads completelycover the opposing ends of the vias. The preferred embodiment of thealignment board further comprises a thin insulating layer attached tothe opposing sides over the board and over the metallic pads, with holesin the insulating layer that coincide with the metallic pads. The holesin the insulating layer are preferably of a small enough diameter not tocover the outside edges of the metallic pads, yet of a diameter greaterthan the ends of the vias. Preferably indium contacts are attached tothe metallic pads inside the openings in the insulating layer. Thediameter of the indium contacts is larger than the diameter of theopposing ends of the vias.

The preferred method of building up the assembly using the alignmentboard is to attach the multi-layer module to the alignment board, andthen attach the infrared detector arrays to the alignment board. Reflowsoldering is preferably used to make the connections, with soldersurface tension preferably playing a part in determining the position ofthe lightweight detector arrays relative to the alignment board.

These as well as other advantages of the present invention will becomemore apparent from the following description and drawings. It is wellunderstood that changes in the specific structure shown and describedmay be made within the scope of the claims without departing from thespirit of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a to 1j are a series of side views illustrating the step by stepprocess in manufacturing the alignment board of the present invention;

FIG. 2 is a perspective view showing the assembly of multi-layer modulesand arrays of infrared detectors to the alignment board;

FIG. 3 is an exploded side view showing assembly of a single multi-layermodule and infrared detector array to the alignment board.

FIG. 3a is and enlarged view of a portion of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The detailed discussion set forth below in connection with the appendeddrawings is intended as description of the presently preferredembodiment of the invention, and not intended the only form in which thepresent invention may be constructed or utilized. The description setsforth the functions and sequence of steps for constructing and operatingthe invention in connection with the illustrated embodiment. It is to beunderstood, however, that the same or equivalent functions and sequencesmay be accomplished by different embodiments that are also intended tobe encompassed within the spirit and scope of the invention.

The manufacturing sequence to build up the alignment board 10 of thepresent invention is illustrated in FIGS. 1a-j, which depict thepresently preferred embodiment of the invention. FIGS. 1a-g show theinstallation of a conductive via 12 with an insulating material sealingplug 14 installed therein. FIG. 1a shows a through-hole preferably laserdrilled in the substrate 16, having a larger diameter on one end thanthe other, typical of holes formed by laser drilling. Other shapes,sizes and configurations of through-holes may be fabricated. FIG. 1bshows application, preferably by screen printing, of a metallo-organicmaterial 18, preferably gold, over the through-hole, while preferablyutilizing vacuum assist to pull the material through the through-hole.FIG. 1c shows that after firing preferably at approximately 850° C., ametallic coating is formed upon the walls of the through-hole. FIG. 1dshows that optionally, after inverting the substrate 16, a secondapplication of metallo-organic material 18 is made in the through-hole.FIG. 1e shows that after a second firing, a second metallic coating isformed on the inside walls of the through-hole. FIG. 1f shows fillingthe via 12 with an insulating material 20, preferably high-temperatureglass. The recommended material is ESL 4905 Special Fine Particle Glassavailable from Electro Science Labs located in King of Prussia, Pa. FIG.1g shows that after firing at approximately 850° C. a sealing plug 14 isformed within the via 12. Refiring the temperature resistant via 12 atup to 550° C. during any subsequent processing has no effect on the via12. Then, excess metallic 18 and insulating 20 material is removed fromboth sides of the substrate 16, preferably by lapping away the excess toform a substantially flush surface on both sides of the substrate 16.The final plugged and lapped surface may be cleaned using an ultrasoniccleaning process.

Next, the remaining operations to make up the alignment board 10 of thepresent invention are performed. FIG. 1h shows both sides of thesubstrate 16 at the vias 12 metallized with metallo-organic material 18,preferably gold. The area of the metallic pad 22 is delineated such thatit is substantially larger than the area of the larger end of the via12. FIG. 1i shows that a thin insulating layer 24, preferably polyamideis added over both sides of the substrate 16. Openings in the insulatinglayer 24 are delineated to form sockets 26 above the metallic pads 22.FIG. 1j shows that a solder pad 28, an alloy of indium with theappropriate melting point, is deposited into each socket 26 on bothsides of the substrate 16. The socket 26 in the insulating layer 24 mustbe shallow enough to allow the bump contacts 30 of the mating parts,multi-layer modules 40 and infrared detector arrays 50 (see FIGS. 2 and3), to contact the indium pads 28. But each socket 26 must be deepenough to contain the indinm pad 28 upon reflow, to prevent shortingbetween the indium pads 28. The alignment board 10 is now ready forassembly.

The method of using the alignment board 10 of the present invention willbe described, to facilitate positioning and attachment of themulti-layer modules 40 to the arrays of infrared detectors 50 as shownin FIG. 2. Preferably, the modules 40 and the detector arrays 50 areattached by conventional reflow solder methods. Reflow soldering is aprocess employed extensively in high-density electronics manufacturing.Solder paste containing a powdered metal alloy dispersed in a liquidmedium, containing an organic solvent flux and a thickening agent, isapplied between miniature electronics components. Then, the solderedjoint is subjected to a sufficiently high-temperature, generally 30°-50°C. greater than the melting point of the alloy in the solder. Theelevated temperature causes the flux and the alloy to liquify andcontact the components, so that upon subsequent cooling the componentsare soldered together.

The multi-layer modules 40 are preferably attached to the alignmentboard 10 first, before the detector arrays 50, by the reflow soldermethods. Then, the detector arrays 50 are attached to the opposite sideof the alignment board 10, also by reflow solder. A lower temperaturesolder 28 is preferably used on the detector side of the alignment board10, so that the multi-layer modules 40 do not become unsoldered duringthe detector array 50 solder operation. The position of the detectorarrays 50 is determined by the locational accuracy of the indium solder28 on the alignment board 10, and also in part by solder surface tensionfrom the soldering operation.

FIG. 3 shows the indium pad 28/bump contact 30 interconnect of thealignment board 10 to the multi-layer modules 40 and the detector arrays50. FIG. 3a shows the structural detail of the alignment board 10 asconfigured at one location to attach to the detector array 50. Theindium pad 28, within the opening in the insulating layer 26, isattached to the metallic pad 22 which in turn is attached to theconductive via 12. An enlarged surface area the size of the indium pad28 is provided to connect to the bump contact 30 of the detector array50. A similar enlarged indium pad 28 the aligned on the opposing side ofthe alignment board 10 to connect to the bump contact 30 of themulti-layer module 40. The detector arrays 50 and the alignment board 10are, in part, self-aligned, because of the balancing of the surfacetension between each respective well defined solder pool of the bumpcontact 30 and the indium pad 28 and the underlying surfaces of thedetector arrays 50 and the alignment board 10.

The alignment board 10 of the present invention may be utilized toposition and attach the multi-layered module 40 to the detector arrays50, without the complicated tooling and glue which was conventionallyused to make the assembly. It is understood that the exemplary alignmentboard 10 shown in the drawing represents only a presently preferredembodiment Of the invention. Indeed, various modifications and additionsmay be made to the preferred embodiment without departing from thespirit and scope of the invention. Thus, modifications and additions maybe obvious to those skilled in the art and may be implemented to adaptthe present invention for use in a variety of applications.

What is claimed is:
 1. An alignment board for interfacing arrays ofinfrared detectors to a multi-layer integrated circuit module, eachlayer having a plurality of conductive conduits formed thereon, thealignment board comprising:a) a board of an insulating material havingfirst and second opposing sides; b) a multiplicity of through-holes inthe board extending between the first and second sides; c) conductivevias formed in the through-holes, the vias having a first end inelectrical communication with an associated detector of one of thedetector arrays, and the vias having a second end in electricalcommunication with an associated conductive conduit; d) sealing plugsformed in the conductive vias, the sealing plugs of an insulatingmaterial that completely fills the vias; and e) a multiplicity of firstmetallic pads attached to the first end of the vias, the first metallicpads sized larger than the first ends of the vias attached thereto. 2.The alignment board of claim 1, further comprising:a) a multiplicity ofsecond metallic pads attached to the second end of the vias forfacilitating electrical communication between the vias and theassociated conductive conduit, the second metallic pads sized largerthan the second ends of the vias attached thereto.
 3. The alignmentboard of claim 2, further comprising:a) an insulating layer formed uponthe first side of the board, the insulating layer having first openingsthat approximately coincide with the first metallic pads.
 4. Thealignment board of claim 3, further including:a) an insulating layerformed upon the second side of the board, the insulating layer havingsecond openings that approximately coincide with the second metallicpads.
 5. The alignment board of claim 3, further including:a) amultiplicity of indinm contacts sized to fit inside the first openingsin the insulating layer, the indinm contacts attached to the firstmetallic pads, to facilitate electrical connection between the firstmetallic pads and the associated detector.
 6. The alignment board ofclaim 4, further including:a) a multiplicity of indium contacts sized tofit inside the second openings in the insulating layer, the indiumcontacts attached to the second metallic pads, to facilitate electricalconnection between the second metallic pads and the associatedconductive conduit.
 7. The alignment board of claim 3, wherein the firstopenings formed in the insulating layer are sized such that the outsideedge of the first metallic pads thereupon are completely covered by theinsulating layer.
 8. The alignment board of claim 4, wherein the secondopenings formed in the insulating layer are sized such that the outsideedge of the second metallic pads thereupon are completely covered by theinsulating layer.
 9. The alignment board of claim 5, wherein the indinmcontacts are sized larger than the first end of the via.
 10. Thealignment board of claim 6, wherein the indium contacts are sized largerthan the second end of the via.